One problem with vertical actuators is that the movable part, which includes the load that is assigned to the actuator, has a certain mass and therefore is subject to gravity. If there is no mechanism for compensating for this gravity, the movable part generally sinks when the actuator is no longer energized. This movable part may either support itself against a lower end stop, or may be held in a low position due to the magnetic attraction capability of the yoke. This magnetic attraction capability of the fixed-part elements, made of magnetic materials, for the movable part equipped with a permanent magnet is referred to as a reluctance force.
In most practical applications, a low position of the movable part which, e.g., has a tool at its lower end, when the actuator is stationary, that is, when it is no longer supplied with electric current, is very unfavorable. Therefore, it is generally preferred to keep the movable part in a high position when the vertical actuator is stationary. Various systems to compensate for gravity are described in the literature.
For example, the vertical actuator described in U.S. Patent Application Publication No. 2006/0061219 is equipped with a device for pneumatic compensation. The movable part has a central spindle which forms a piston that moves in a stationary cylinder in which compressed air is provided at the bottom. This compressed air exerts an upwardly-directed thrust on the lower surface of this central spindle which allows compensation for gravity and makes it possible to have a pre-defined stable position when the actuator is not being energized. Such a system has various disadvantages. First of all, a relatively bulky, additional device is needed for the weight-compensation function. In addition, it is also necessary to provide a device for ensuring correct movement of the central spindle in the cylinder. Moreover, the feeding of compressed air and its monitoring requires an additional device and the mastery of an additional technology. Such a system is thus relatively complex and costly. Furthermore, it should be noted that the pressure acting on the central spindle must be exerted from below this spindle. Therefore, the area below the spindle is obstructed by the compressed-air device which is disposed at the lower end of the cylinder in which the central spindle of the movable part moves. Consequently, the movable part is unable to bear a load at the bottom of the central spindle. Incidentally, this actuator is provided in order to bear a load above its movable part. This seriously limits the practical applications to be considered.
FIG. 1 is a schematic, vertical cross-sectional view of a conventional actuator of a type which is considered, e.g., within the framework hereof. The description of this vertical actuator 2 and its magnetic performance characteristics illustrated in FIG. 2 makes it possible, based on this precise case, to demonstrate the problem of the conventional systems for which effective solutions are provided as explained below with reference to FIGS. 3 to 5.
Actuator 2 includes a stationary part 4 formed by a cylindrical yoke 8 made of magnetic materials and a coil 10. The coil 10 includes a lower part having a winding implemented in a first direction and an upper part having a winding implemented in a second direction, opposite the first direction. This coil 10 therefore defines a single-phase actuator which, however, is disposed such that the magnetic field generated by the lower part has a direction which is opposite that of the magnetic field generated by the upper part of this coil. This manner of implementing the winding of coil 10 should be familiar to those skilled in the art. Movable part 6 of actuator 2 includes a permanent magnet 14 having axial magnetization, which is mounted on a central spindle or shaft 16 that defines a central geometric axis 18. Two magnetic disks 20 and 21, respectively, are disposed at the two ends of the magnet. These disks make it possible to channel the magnetic flux in the direction of coil 10. Together with magnet yoke 8, they form a magnetic circuit for the magnetic flux, which therefore is coupled to coil 10 when the latter is supplied with electric current.
With the arrangement of coil 10 as illustrated, the functional range provided for this actuator is between, e.g., 0 and 12 mm, as shown on axis Z. Two curves are illustrated in FIG. 2. First upper curve 24 represents the reluctance force as a function of the position of movable part 6 along vertical axis Z. The position of the movable part is defined by the position of the median plane of magnet 14. Thus, in FIG. 1, movable part 6 is illustrated in the 0-position on axis Z, i.e., in the lower position of the functional range provided. It can be seen that coil 10 is shifted slightly downward in relation to the median plane of magnet yoke 8. This median plane corresponds to the 7 mm position. When movable part 6 is located in the 7 mm position, the median plane of permanent magnet 14 thus coincides with the median plane of yoke 8. Because of the symmetry of the resulting configuration of the magnetic elements of actuator 2, the reluctance force exerted by the stationary part on the movable part is equal to zero in this position, as illustrated in FIG. 2. When movable part 6 drops, the reluctance force exerted on the movable part is positive and increases with the distance in comparison to the center position of the yoke. When the movable part moves beyond this center position, the reluctance force becomes negative. Curve 24 thus represents an asymmetry in relation to the median plane of yoke 8, which lies at a level that corresponds to the 7 mm position on vertical axis Z.
Curve 26 represents the resultant force which is exerted on movable part 6 in response to the absence of an electrical supply to coil 10. In the case of actuator 2, a gravitational force of, e.g., 2 N, is provided. Curve 26 is thus similar to curve 24, however shifted downward by a value of 2 N. Therefore, it can be seen that movable part 6 has a position of stable equilibrium approximately in the 2.5 mm position on axis Z. The profile of resulting curve 26 has several disadvantages, which have already been described at least partially above. The position of stable equilibrium of the movable part upon absence of a current supply corresponds to a low position in the functional range of actuator 2. In the event the movable mass includes a tool at its lower end, this tool remains in a low position or drops into such a low position when the current supply is interrupted. In certain instances, this can cause damage to the tool or to the product being produced at the moment, which is situated under this tool. In addition, it can be seen that the slope of curve 26 is relatively small in the vicinity of the equilibrium point. Therefore, the restoring force in the vicinity of the equilibrium position is small, such that the stability of the movable part is small when there is no current supply. Moreover, it can be seen that resultant force 26 decreases monotonically over the functional range of the actuator. The reluctance force exerted on the movable part varies over the entire functional range. Resultant force 26 has a not insignificant value in the high positions of movable part 6.